1. Field of the Invention
The present invention relates to an apparatus and method for estimating the moving speed of opposing stations in mobile communications system.
2. Description of the Related Art
Conventionally, for a channel multiplex method in mobile communications system, a time division multiple access (TDMA) system, frequency division multiple access (FDMA) system, etc. have been used. However, the development of a method with a more efficient use of frequency is desired, and particularly, a direct sequence code division multiple access (DS-CDMA) system is a promising candidate, since a large volume of communications are available with this system.
The DS-CDMA system is one kind of spread spectrum communication system. In this system, on the transmitting side data signals are transmitted with the spread spectrum by using the same frequency for a plurality of channels and multiplying the data signals by an independent spread code with a broad band for each channel, and on the receiving side the data signals for each channel are restored by multiplying received signals by the same spread code. The multiplication of a spread code on the receiving side is called a despread. When this DS-CDMA system is applied to mobile communications, a searcher function, transmission power control function, absolute coherent detection function, etc. are indispensable.
A searcher function means a function for detecting a transmission path and a despread code timing being a timing for performing a despread. A transmission power control function means a function for modifying a transmission power for near-far problem due to the difference in distance between a mobile station and a base station, and an instantaneous fluctuation (fading) due to multi-paths. A coherent detection function means a function for adding a pilot signal to data signals in order to obtain a required bit error rate (BER) in a lower transmission power when transmitting, and performing a coherent detection.
In mobile communications, a stable communication is required in such a variety of dynamically changing environments that a mobile station may transit from a stationary status to a high-speed status, and a mobile station may transit from an urban environment to a suburban environment, etc. Particularly, in a multi-path environment accompanied by reflected waves and delayed waves passing through a plurality of transmission lines, countermeasures are indispensable, since fading (instantaneous value fluctuation) is generated by interference. In the DS-CDMA system too, effective fading countermeasures are desired in connection with each of the above-mentioned functions.
However, there are the following problems in mobile communications using the DS-CDMA system.
Generally speaking, although there are optimal values for parameters of each unit of a communication apparatus against a fading generated during the communication, each parameter is not always set to the optimal value, since the fluctuation speed of the fading changes depending on the moving speed of the mobile station (or fading pitch). Therefore, when the parameters are not optimized, degradation is generated in the receiving characteristics, and thereby the channel capacity is reduced. It is necessary to estimate the moving speed of the mobile station at the base station in order to optimize each parameter.
However, in the DS-CDMA system, since a plurality of channels are multiplexed on the same frequency, unlike a TDMA or FDMA system which are well known as other multiplex methods, it is difficult to estimate the moving speed based on the measurement of the receiving field strength.